The present invention relates to liquid metering. More particularly, the invention provides a device capable of registering very low flows, being particularly useful in the laboratory and in medical applications, including intravenous infusions, blood flow control and urine output measurement.
Flow meters for liquids have many important applications such as furnishing information in medical applications, research, industrial and agricultural work. Known types of flow meter include the nutating-disk, lobed impeller, office flow obstruction, tapered tube rotameter, turbine, and magnetic types. The state of the art in flow meter development is represented by recent US Patents, among them U.S. Pat. Nos. 5,571,964 and 5,581,026 to Sawada et al., and U.S. Pat. No. 5,698,793 to Carmichael. Conventional types of flow meter become unreliable for low flows, and for very low flows which may be discontinuous no meaningful results are obtained. Yet there is a need for such measurements in applications such as for example for metering the urine output of some critical hospital patients.
In many hospitals it is common that the average urine flow is determined hourly by having a nurse or attendant examine the transparent graduated collection bag located below the bed of the patient. In intensive care departments such personnel are likely to be distracted by more urgent tasks and omit to take such measurements. The method is costly, unreliable and due to the flexible collection bag, also inaccurate.
Many special devices have been proposed. Among patent disclosures are devices for rainfall measurement and meters which are either intended for urine flow measurement or can be adapted for such purpose.
Dye et al. in U.S. Pat. No. 3,859,854 disclose an apparatus for measuring a liquid discharge, which includes a receptacle including a wall, a cup shaped pan, and a chamber below the pan. Liquid is accumulated in a compartment, and the height of the liquid is measured.
Wurster proposes a capacitance type of measurement for urine flow in U.S. Pat. No. 4,051,431. Using the urine as an electrical conductor and as a dielectric, the capacitance is used to derive the volume of urine present.
Nehrbass in U.S. Pat. No. 4,099,412 proposes to use a rotameter to measure the instantaneous flow rate of urine discharge.
In U.S. Pat. No. 4,484,582 Rottenberg et al. disclose a system for measuring flow of electrolytic fluids, wherein a pair of electrodes are flush with the surface of a cylindrical flow cell. A monopolar pulse train is applied across the electrodes to effect a cell impedance inversely proportional to the flow rate.
Jespersen describes optical sensor means to operate a lower and an upper valve described in U.S. Pat. No. 4,343,316. Intermittent discharge of a measured volume is used to indicate urine flow rate.
Parrish discloses an ultrasonic transceiver in a measuring system he describes in U.S. Pat. No. 4,448,207. The transceiver is arranged to periodically measure the height level of a urine column.
LeVeen discloses a meter based on an optical sensor in U.S. Pat. No. 4,532,936. A peristaltic pump is used to empty the measurement column at a known rate.
Carter et al. describes a pressure sensor in a funnel-shaped device described in U.S. Pat. No. 4,554,687. A pressure transducer produces an electrical signal in response to air pressure resulting from urine accumulation. He further details a second variation using a sealed vertical air column in U.S. Pat. No. 4,683,748.
A device intended for flow measurement of small liquid volumes is disclosed by Prestele in U.S. Pat. No. 4,559,831. He channels the liquid through a small diameter tube into which gas bubbles are also fed. By measuring the advance of the bubble, fluid flow rate can be deduced.
Westphal et al. suggest a force transducer to weigh collected urine in an apparatus described in U.S. Pat. No. 5,769,087.
Nelson proposes devices to measure rainfall by electronic drop counting in U.S. Pat. No. 4,520,667 and 4,827,766. The latter patent discloses a vent tube which is partially liquid immersed.
Most flow measuring systems measure some related parameter to deduce flow rate. However errors are introduced when the relationship between the measured parameter and the flow rate is not constant. Such relationship is often effected by temperature changes, and by changes in the fluid density and viscosity. Few prior-art systems are capable of measuring extremely low flows, such as a flow of a few drops per hour.
It is therefore one of the objects of the present invention to obviate the disadvantages of prior art flow meters and to provide an apparatus which is able to record flow of a few drops per hour.
It is a further object of the present invention to provide a flowmeter which meets the requirements for registering urine output of catheterized patients, and also for metering infusion rates.
The present invention achieves the above objects by providing a low-flow metering device for measuring the volume of an amount of liquid exceeding 0.05 ml., comprising:
a) a first chamber having an inlet, and an outlet in fluid communication with a second chamber, said first chamber containing a flow restriction element creating a laminar flow and said outlet of said first chamber being provided with a drop generator leading to said second chamber and sized to release a series of droplets, each of said droplets forming and breaking away under its own weight from liquid in the drop generator orifice fed thereto by said flow restriction element,
b) an overflow conduit extending between an upper area of said first chamber and a lower area of said second chamber for equalizing gas pressure between said two chambers;
c) electronic means positioned in said second chamber below said drop generator for counting the passage of each droplet exiting therefrom; and
d) an information processing unit connected to said electronic means for receiving and recording information and for measuring the time interval between successive droplets and for calculating flow rates and total volume therefrom while dealing with variations in droplet size as a function of the time interval between successive droplets.
In U.S. Pat. No. 3,641,543 (D1), there is described a low-level detector and drop rate monitor, particularly for intravenous feeding apparatus. This patent primarily relates to the electronic means of detecting the drops and fluid to be monitored and relates only briefly to the mechanical apparatus which is of little importance to the invention described in said patent. While the drawing in FIG. 1 in said patent, shows the provision of an air line 11, in fact said patent does not teach or suggest the device of the present invention to a person skilled in the art since the device as shown in said figure, does not work. Specifically, since air line 11 extends into an inverted bottle which has no inlet, as the liquid flows therefrom a vacuum is created in the upper portion of container 12 which will balance against the force of gravity and not permit complete flow of the liquid from said bottle. Furthermore, valve mechanism 20 is separate from the drop formation opening and if said valve is closed the drops will fall until they fill the second chamber whereby electrodes 38 and 40 will be unable to measure any drop formation. Finally, said patent does not teach or suggest the arrangement of the present invention, wherein the flow restriction element leads directly to the drop generator and instead in said patent, and in other patents discussed hereinafter, the flow restriction element 20 is in fact positioned in the line after the drop generator. Therefore, said patent neither teaches nor suggests the present device.
U.S. Pat. No. 5,098,408 (D2), teaches the use of an automatic intravenous flow controller wherein the primary purpose is to restrict the flow in a controlled way. In said patent, drops are formed along the outer surface of an icicle or stylus drop former positioned in fluid communication between an inlet and outlet end of the flow controller. These drops break away from the material of the icicle and the surface tension between the icicle and the fluid determines the size of the drops. The drops are not counted and the drops only serve as a rough visual indication of flow rate. In contradistinction, according to the present invention, the drops are formed within the drop generator and each droplet breaks away under its own weight from liquid in the drop generator orifice, i.e. by the breaking of liquid-liquid surface tension.
U.S. Pat. No. 4,520,667 (D3), teaches a non-mechanical digital rain gauge. As can be seen from the description in column 5 of said patent, the device is designed in such a manner that there will never be more than 2 rain drops entering the opening thereof in a second. Therefore, no flow restriction device is required or taught and each drop that enters the device passes into the drop generator and is counted.
U.S. Pat. No. 3,871,229 (D4), teaches a drop sensing apparatus with a flow restriction device positioned along the tube feeding the needle. While flow may be corrected by said flow restriction device, as a result of the drop rate, this patent neither teaches nor suggests an arrangement wherein the flow restriction element leads directly to the drop generator.
In U.S. Pat. No. 3,712,132 (D5), there is described a droplet monitoring probe which comprises 2 electric wires spaced a known distance apart from each other and connected at one end to means for establishing a DC potential between the wires. A drop in the fluid stream momentarily contacts both wires causing an electrical signal, which may be counted. These droplets, however, are not falling droplets and instead, are droplets forming and flowing in a gas turbine.
In U.S. Pat. No. 4,261,388 (D6), there is described a drop rate controller for controlling flow of fluid from an infusion fluid reservoir to an intravenous infusion site apparatus with a flow restriction device positioned along the tube feeding the infusion site. While flow may be corrected by said flow restriction device, as a result of the drop rate, this patent neither teaches nor suggests an arrangement wherein the flow restriction element leads directly to the drop generator.
Thus it will be realized that none of said patents teaches or suggests a low-flow metering device as described and claimed herein, wherein the flow restriction element leads directly to the drop generator and wherein there is provided means for measuring the time interval between successive droplets and for calculating flow rates and total volume therefrom while dealing with variations in droplet size as a function of the time interval between successive droplets.
It will be understood that the novel device of the present invention. serves to measure flow even when fluid supply is non-continuous. As even a single droplet is registered by the electrodes or optical means, and there is no practical limit to the time span between one droplet and the next, very low flow rates can be reliably metered. Applications include infusions, urine output, rainfall measurement, patient""s blood input or output, patient feeding with liquids, and determining the results of overhead irrigation in agriculture.
The invention provides means for measuring the time interval between successive droplets, and calculates flow rates therefrom. It is realized that the size of the droplets may change in relation to said time interval. Means for dealing with variation in droplet size are provided as will be seen with reference to FIGS. 2a and 2b. 
Furthermore, an embodiment of the invention, described with reference to FIG. 7 is able to recognize very fast inflow causing overflow and to report such condition, as well as being able to report a full collection bag, and to activate an alarm if needed.
The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.